1. Field of the Invention
The present invention relates to communication equipment and, more specifically, to equipment for wireless local area networks (WLANs).
2. Description of the Related Art
A typical WLAN system may include one or more stations (STAs), e.g., a cell phone, a laptop computer, and/or a hand-held computer, each of which is equipped with a generally available WLAN PC card. WLAN PC cards enable STAs to communicate among themselves (i) directly, when located within the same service area, and/or (ii) indirectly, through a network server, when located in different service areas. The network server provides support for communication between STAs in different service areas through the access points (APs) located in those service areas. A basic service set (BSS) is formed between the STAs or between an AP and one or more STAs within the same service area.
One example of a WLAN network is a network that conforms to standards developed and proposed by the Institute of Electrical and Electronic Engineers (IEEE) 802.11 Committee (referred to herein as a network operating in accordance with the IEEE 802.11 family of standards), which standards are incorporated herein by reference. In an 802.11 WLAN network, all messages transmitted among different STAs of the same service area are typically transmitted via the AP rather than being transmitted directly between the STAs. Such centralized wireless communication provides significant advantages in terms of simplicity of the communication link as well as in power savings. However, if necessary or appropriate, an 802.11 WLAN network can also be configured to transmit messages directly between two STAs in a so-called IBSS (independent basic service set) mode.
Most WLAN networks are organized as a series of layers (a layered-network architecture), each layer built upon its predecessor. The purpose of each lower layer is to offer services to the higher layer(s) and, at the same time, shield those higher layers from implementation details of the lower layer(s). Between each pair of adjacent layers is an interface that defines those services. The lowest layers are the data link and physical layers. The function of the data link layer is to partition input data into data frames and transmit the frames over the physical layer sequentially. Each data frame includes a header that contains control and sequence information for the frames. The function of the physical layer is to transfer information over a communication medium.
FIG. 1 shows a prior-art framing sequence for user data in a representative 802.11-compliant WLAN. More specifically, six protocol layers are shown in FIG. 1: an application layer 150, a transmission control protocol (TCP) layer 151, an Internet protocol (IP) layer 152, a logical link control (LLC) layer 153 (a sub-layer in the data link layer), a medium access control (MAC) layer 154 (also a sub-layer in the data link layer), and a physical (PHY) layer 155. User data 101 are provided to application layer 150, which generates application data 102 by appending an application-layer header 110 to the user data. Application data 102 are provided to TCP layer 151, which appends a TCP header 111 to application data 102 to form a TCP segment 103. IP layer 152 appends an IP header 112 to TCP segment 103 to form an IP frame 104. IP frame 104 might be a typical TCP/IP packet that is commonly employed in many data networking applications including some that are not necessarily 802.11-compliant. LLC layer 153 provides a uniform interface between MAC layer 154 and the higher layers, thereby providing transparency of the type of WLAN used to transport the TCP/IP packet. LLC layer 153 appends the interface information as an LLC header 113 to IP frame 104 to form an LLC frame 105.
In an 802.11-compliant WLAN, the physical device is a radio and the physical communication medium is free space. A MAC device and a PHY-layer signaling control device ensure that two WLAN terminals are communicating with the correct frame format and protocol. The IEEE 802.11 standard for WLANs defines the communication protocol between two (or more) peer PHY devices as well as between the associated peer MAC devices. According to the 802.11 WLAN data communication protocol, each packet frame transferred between the MAC device and the PHY device has a PHY header, a MAC header, MAC data, and error checking fields.
A typical format for the MAC-layer frame of an 802.11-compliant WLAN system appends a MAC header 114 and a frame check sequence (FCS) 115 to LLC frame 105 to form a MAC frame 106. MAC header 114 includes frame control, duration identification (ID), source and destination addresses, and data sequence control (number) fields. The data sequence control field provides sequence-numbering information that allows a receiver to reconstruct a user data stream from a sequence of MAC frames. PHY layer 155 forms a physical-layer packet frame 107 by appending a PHY header 118 to MAC frame 106. PHY header 118 includes a preamble 116 and a physical-layer convergence protocol (PLCP) header 117. PLCP header 117 identifies, for example, the data rate and length in PHY layer 155, and preamble 116 might be used by a receiving device to (i) detect/synchronize to the incoming frame and (ii) estimate the channel characteristics between the transmitter and receiver.
The effective data throughput or efficiency of the IEEE 802.11 WLAN protocol is impacted by several sources of overhead. For example, the overhead can originate at the PHY layer (e.g., preamble 116 and PLCP header 117) and the MAC layer (e.g., MAC header 114, FCS 115, acknowledgement (ACK) frames, inter-frame spacing (IFS), and contention overhead). Some improvements in the efficiency have been achieved in the IEEE 802.11e enhancements for Quality of Service (QoS) by incorporating mechanisms for frame bursting and block acknowledgement (Block-ACK), which tend to reduce the overhead associated with ACK frames, the IFS, and contention. However, it may still be desirable to further reduce the overhead by targeting other overhead sources.